Foam-making installations

ABSTRACT

The present invention relates to an installation for making foams which comprises a blender comprising an annular mixing chamber of revolution through which said mixture circulates from the upstream end to the downstream end thereof, said annular mixing chamber being bounded by two respectively outer and inner coaxial walls preferably bearing protuberances thereon and possessed of relative rotating motion. The outer and inner walls are respectively subjected to the actions of two separate cooling circuits each equipped with individual means for adjusting its effectiveness, said two cooling circuits and the individual adjustment means thereof being so devised as to enable said outer and inner walls to be maintained substantially at the same temperature within the full speed range of said blender, said temperature being lower than a critical index temperature above which the physical properties of the mixture are affected.

United States Patent [72] Inventor Henri .1. Bourne Trept, France 211 Appl. No. 747,891 [22] Filed July 26, 1968 [45] Patented Nov. 30, 1971 [73] Assignee Tunzini Amelioraci Paris, France [32] Priorities July 27, 1967 [33] France [31 115995;

July 2. 1968, France, No. 157553 [54] F OAM-MAKING INSTALLATIONS 10 Claims, 6 Drawing Figs.

[52] U.S. Cl 252/359 E, 165/88. 165/92 [51] 1nt.C1 ..B0ld,B0lf,

B01 j 13/00 [50] Field of Search 252/354.3-354.5; 259/7, 9.10, 27, 45; 165/88, 92; 239/343; 264/50; 260/2, 5 ALB AX, 5 L, 5 E, 5

[56] References Cited UNITED STATES PATENTS 1.897.613 2/1933 Jensen 165/72 2,266,032 12/1941 Harrington 62/342 2,278,340 3/1942 Weinreich et al 6001 4 N7 RETURN 3 Primary Examiner- Norman Y udkoff Asxislanl Examiner-J. Sofer Almrney- Larson. Taylor & Hinds ABSTRACT: The present invention relates to an installation for making foams which comprises a blender comprising an annular mixing chamber of revolution through which said mixture circulates from the upstream end to the downstream end thereof. said annular mixing chamber being bounded by two respectively outer and inner coaxial walls preferably bearing protuberances thereon and possessed of relative rotating motion. The outer and inner walls are respectively subjected to the actions of two separate cooling circuits each equipped with individual means for adjusting its effectiveness, said two cooling circuits and the individual adjustment means thereof being so devised as to enable said outer and inner walls to be maintained substantially at the same temperature within the full speed range of said blender, said temperature being lower than a critical index temperature above which the physical properties ofthe mixture are affected.

PATENTED uuvso 19" SHEEI1UF3 Leon/WT PUMP FIQ

PLAST/c LCM/ANT FOAM-MAKING INSTALLATIONS The present invention relates to foam-making installations, the word foam" being here taken in a very broad sense to include all open-cell products, regardless of their hardness or flexibility or of their end-utilization (for industrial, household, food, or other uses), such products being usually obtained by miting natural or synthetic ingredients and incorporating air into the mixture. The invention relates more particularly though not exclusively to installations of this kind more specifically intended for making foams based on such plastics or polyvinyl chloride, polyurethane, polyethylene, acryloyls, copolymers, synthetic or natural latex, etc.

The present invention has for its object to provide an installation of this kind so devised as to enable the quality and notably the homogeneity of the foam obtained to be improved.

An improved installation according to the invention includes, for the purpose of incorporating air in the foam-making mixture, a blender comprising an annular mixing chamber of revolution through which said mixture passes from the upstream end to the downstream end and which is bounded by coaxial outer and inner walls preferably provided with protrusions thereon and possessed of relative rotational motion, and it is the principal teaching of the present invention that said outer and inner walls are subjected to the respective action of two separate cooling circuits each comprising individual means for adjusting its effectiveness, these two cooling circuits (including their individual adjustment means) being so devised as to enable said outer and inner walls to be maintained at substantially the same temperature within the limit operating conditions of the blender, this temperature being lower than a set temperature above which the physical properties of the mixture are adversely affected.

The description which follows with reference to the accompanying nonlimitative exemplary drawings will give a clear understanding of how the invention can be carried into practice and will reveal yet further particularities and features thereof which naturally fall within the scope of the invention.

In the drawings:

FIG. I shows schematically in axial section a first possible embodiment of a blender for making a plastic foam.

F lGS. 2 and 3 show on an enlarged scale two detail views of FIG. I. And

H08. 4 to 6 show correspondingly to FIG. 1 three altemative forms of embodiment of said blender.

In what follows it will be assumed that the installation according to the present invention is intended for producing a foam based on one or more plastics and that the synthetic foams to be made are of the kind produced in the form of more or less thick supple sheets or layers with a microcellular structure, for subsequent utilization in the manufacture of a variety of synthetic products intended for use notably as floor or wall coverings, linings, protective or reinforcing elements, and so forth, all of which require a uniform surface aspect and a homogeneous constituent material.

Now experience has shown that obtaining a uniform and homogeneous foam raises difficult production problems connected with questions of temperature, since a nonuniform temperature in the plasticsbased foam issuing from the production installation will result in correspondingly heterogeneous physico-chemical characteristics in the individual fluxes at different temperatures constituting the overall flux.

These elemental fluxes with different physico-hemical characteristics consequently have properties which differ likewise in behavior when transfers are made into a mould or when a coating operation is carried out on a support, the effect of which is to impart an irregular surface aspect and a heterogeneous structure to the end product.

These drawbacks are all the more serious in that it is impossible to overcome them by stirring the foam flux issuing from the production installation since this would tend to eliminate the minute bubbles embodied in the foam, causing the latter to lose its fundamental characteristic, namely that of being a solid with a cellular structure.

It is a principal object of the present invention to overcome the aforementioned drawbacks by providing an installation enabling a homogeneous plastic foam of uniform surface aspect to be obtained.

As is clearly shown in FIG. 1, an installation according to the present invention includes, in addition to other component parts well-known per se, a blender of revolution 1 comprising an annular chamber 1a through which flows a mixture under pressure of air and a plastic which are fed beforehand thereinto. The coaxial walls of said chamber are possessed of rotary motion relatively to each other, and this can be accomplished (as will be assumed hereinafter) by devising the outer wall 1b of annular chamber la in the form of a stator and the inner wall 1c thereof in the form of a rotor rotated by an electric motor 2.

Delivery under pressure into annular chamber la of the ingredients to be mixed takes place as follows The plastic is injected by means of a pump 3 which generates a pressure of the order of 5 atmospheres and the delivery conduit 3a of which has port in an annular manifold 4 located at the upstream end of annular chamber In.

The air to be incorporated into said plastic is supplied by an injector 5 delivering pressurized air into said delivery conduit 3a of pump 3 at a pressure (of 6 atmospheres for example) somewhat higher than that of the plastic flowing therethrough.

The foam produced in annular chamber la of blender l is discharged through an outlet conduit 6 connected to an annular manifold 7 positioned at the downstream end of said chamber, and this outlet conduit 6 deliveries the foam produced to a device for moulding the end-product or for depositing it in the form of a coating, for instance.

With a view to improving blending of the mixture passing through annular chamber 1a, the facing surfaces of outer and inner walls lb and 1c of annular chamber la may be provided respectively with protuberances 8 of complementary shape, such as the tetrahedral shape shown on an enlarged scale in FIG. 2 or the parallelepiped shape shown in FIG. 3, whereby, by virtue of their relative motions, these two sets of protuberances assist homogenization of the mixture and uniform distribution of the minute air bubbles therein.

lt would seem opportune at this stage of the exposition to point out that if no further measures were to be taken, firstly, the friction engendered within the mixture (chiefly due to the blending effected by the protuberances 8) would soon lead to a rise in the temperature of the mixture above the pregelling temperature thereof, thereby rendering the installation inoperative due to a global setting of the mixture in annular chamber 1a, and, secondly, during the brief period of normal operation of the installation, there would be noted at the outlet of delivery conduit 6, a physical-chemical heterogeneity of the foam discharged and a consequent irregular surface aspect of the end-product produced in the device for shaping said foam.

Indeed, unless special precautions are taken, the foam being produced in blender 1 would exhibit the temperaturewise heterogenuty referred to above owing to the fact that the overall temperature rise resulting from operation of the installation would affect the rotating inner wall It more than the stationary outer wall lb, the temperature differential between said inner and outer walls increasing with increasing rotor speed and resulting, in the mixture being produced, in radial temperature gradients decreasing from inner wall 10 towards outer wall lb.

In other words, the mixture being produced would exhibit a veritable stratification, with different strata exhibiting different temperatures and hence different physico-hemical characteristics.

This being so, it is obviously useful, not to say virtually essential, to take special precautions designed to prevent, on the one hand, an undue temperature rise in the flux circulating through blender chamber la, and, on the other hand, the temperaturewise heterogeneity referred to above.

It is accordingly the principal teaching of the present invention that stationary outer wall lb and rotating inner wall 1c of the annular chamber la of blender l are subjected to the efiects respectively of two separate cooling circuits;

said two cooling circuits include separate means for adjusting their effectiveness; and

said cooling circuits and their associated individual adjustment means are so devised as to enable outer wall lb and inner wall lc to be maintained, within the range of operating conditions of the blender, substantially at the same temperature T, below a limit temperature T (to which reference will be made in greater detail hereinafter) above which the physical properties of the synthetic foam being made are adversely affected.

To this end, recourse may be had with advantage to the form of embodiment shown in FIG. 1, in which the cooling system includes a cold-generating unit 9 supplying a cooled fluid (glycol-bearing water, for instance) to a pump 10 the delivery conduit 10a of which supplies said fluid in parallel to the two cooling circuits respectively associated to outer wall lb and inner wall 1c;

the cooling circuit of outer wall lb is formed by a feed conduit ll branching off delivery conduit 10a of pump 10 and extending up to the downstream end of a cooling duct which lines outer wall lb and consists, for instance, of a helicoid 12a formed within said outer wall and terminating, at the upflow end of the blender, at a discharge conduit 13 which returns the cooling fluid to a recycling manifold 14 connected to the inlet of cooling unit 9, said cooling circuit being provided with a flow regulating valve 15 preferably connected into feed conduit 11; and

the cooling circuit of inner wall lc is formed by a feed conduit l6 likewise branching off delivery conduit 10a of pump 10 and extending up to the downstream end (made hollow by design) of blender rotor shaft 17 and thence, via a first set of holes 180, to a cooling passage 12b provided on the inside surface of inner wall lo and extending, via a second set of holes 18b, to the upstream end (likewise made hollow) of said shaft 17 and thence, via a discharge conduit 19, to recycling manifold 14, said cooling circuit being equipped with a flow regulating valve 20 preferably connected into feed conduit 16.

It will readily be appreciated that with such an installation it will suffice for the operator to appropriately set the two cooling circuit adjustment valves 15 and 20 to maintain the temperature T, of the foam being produced below the critical temperature T However, in order to enable him to properly set adjustment valves 15 and 20, the operator must be provided with certain data concerning the different operating parameters of the installation, and to this end the latter preferably includes measuring instruments or gauges, and notably the following as schematically illustrated in FIG. 1:

A thermometer 21 to indicate the foam temperature on exit from the blender.

A thermometer 22 to indicate the temperature of the cooling fluid in discharge conduit [3 of the cooling circuit of outer wall lb.

A thermometer 23 to indicate the temperature of the cooling fluid in discharge conduit 19 of the cooling circuit ofinner wall lc.

A thermometer 24 to indicate the temperature of the cooling fluid in delivery conduit 10a of pump 10.

A flowmeter 25 to indicate the cooling fluid flow rate through said delivery conduit 10a. And

An ammeter 26 to indicate the load on motor 2 driving blender lv With regard to the critical index temperature T to be observed, allowance must be made in setting it for the fact that the higher the speed of the blender rotor and the greater the viscosity of the mixture being processed. the more the working temperature will tend to rise.

Further, the index temperature must of course make allowance for the chemical nature of the foam, and notably for the natures of the resin used and the stabilizing agent incorporated in the foam, for the homopolymer and copolymer resins usually employed for making plastic foams have a pregelling temperature above which they set bodily, thus making it obviously necessary to adopt an index temperature T (and a fortiori an operating temperature T,) below this progelling temperature.

To fix ideas, it may be stated that certain homopolar resins have a pregelling temperature of about 35 C. whereas some copolymer resins have a pregelling temperature usually lying between 28 and 35 C. and in certain exceptional cases as low as 22 C.

As for the stabilizers used (usually soaps such as potassium oleate or methyl laurate, or a synthesized wetting agent of the alkylarysulfonate type), they are severally effective only with clearly specified temperature limits, so that it is obviously necessary to choose a stabilizer whose lower limit of effectiveness lies below the pregelling temperature of the resin employed.

In any event, for a given mixture (of resin and stabilizer), the operating temperature T, adopted must be lower than the pregelling temperature of the resin and must lie within the limits of effectiveness of the stabilizer, the index temperature T therefore being a temperature which lies within those same limits of effectiveness of the stabilizer and is lower than the pregelling temperature of the resin.

ln point of fact most stabilizers have effectiveness envelopes the upper limits of which are approximately the same as the pregelling temperatures, and in this connection mention may be made of the stabilizers known under the trade-names Fomade D" (stable up to about 28 C.), "Fomade B" (stable up to about 3 I C.) and Cellset l5 (stable up to about 24 C.

As an illustration of the inherent possibliities of an installation according to the present invention, it is now proposed to give two examples of foams capable of being made with such an installation. In both examples the foam pressure is 5 atmospheres and the pressure of the air to be incorporated in the foam is 6 atmospheres. Further, in both examples the stabilizer adopted is selected for its effectiveness at the working temperature.

EXAMPLE 1 This example relates to a polyvinyl plastisol foam consisting ofa 50 percent plasticized standard homopolymer resin stabilized by Fomade D. In this example the index temperature T is the pregelling temperature of the plastisol, i.e. about 30 G, minus a small safety margin.

When the rotor speed is 250 rpm. and the temperature of the glycol-bearing water is 8 C., the operating temperature T, is 24 C., the foam obtained exhibiting great uniformity.

[n a variant of this example, in which Fomade B" is used as the plasticizer and a rotor speed of 350 rpm, is adopted, the temperature of the glycol-bearing water must be maintained at -5 C. in order to obtain an operating temperature T, of the order of 26 C. to 27 C., that is to say below the index temperature T which remains at 30 C.

EXAMPLE 2 This example relates to a polyvinyl plastisol foam consisting of a 50 percent plasticized copolymer resin stabilized by Cellset l5" and having a low gelling point. In this example the index temperature T is the pregelling temperature of the plastisol, i.e. about 25 C., reduced slightly.

When the rotor speed is 300 rpm. and the temperature of the glycol-bearing water is -8 C., the operating temperature T, is 22 C. and the foam obtained again exhibits great uniformity.

No reference has been made in the foregoing to the profile of annular chamber 1 in the axial sense.

Whilst it is possible in some cases to adopt a cylindrical annular chamber, it would appear to be preferable in most cases to adopt an annular chamber of tapering form in which the mixture is uniformly accelerated in the divergent sections and uniformly retarded in the convergent sections thereof, cylindrical interconnecting sections therebetween being preferably provided in which the speed of the mixture consequently remains uniform.

Generally speaking, steps are taken so that the mixture is accelerated in the upflow portion of the annular chamber and retarded in the down flow portion thereof.

Reference is now had to the exemplary embodiment shown in FIG. 1, in which the annular chamber of the blender is shown as comprising, from its upstream end to its downstream end, a divergent section followed by a convergent section obtained by joining the larger ends of two cone frustums. Such a chamber configuration is suitable for polyvinyl chloride or natural latex foams.

As stated precedingly, however, it is preferable in many cases to cause the blender to include cylindrical interconnecting sections within which the mixture circulates at uniform speed.

In the exemplary embodiment depicted in FIG. 4, a blender 1 comprises, from its upstream end to its downstream end, a divergent section t,, a cylindrical section t a divergent section t a convergent section t.,, a cylindrical section t and a convergent section t,,.

In the alternative exemplary embodiment shown in FIG. 5, the blender 1 comprises, from its upstream end to its downstream end, a divergent section t,, a convergent section t.,, a cylindrical section t and a convergent section t Lastly, in the exemplary embodiment portrayed in FIG. 6, the blender 1 comprises, from its upstream end to its downstream end, a divergent section t,, a cylindrical section t,, a divergent section t a cylindrical section t,,, a convergent section t;, a cylindrical section t,; and a convergent section t,.

From the dimensional standpoint, the axial length H of each section (whether frustoconical or cylindrical) will be determined by the formula:

where T is the blending time, D the foam flow rate, and S the annular cross section of the chamber 1 (or the mean annular section in the case of frustoconical sections).

For indicative purposes, it may be stated that a blender of the kind shown in FIG. 4 is suitable for making plastisol or acrylic synthetic latex foams, whereas a blender of the kind shown in either of FIGS. 5 or 6 is suitable for making a natural latex foam.

It will be manifest that irrespective of the form of embodiment adopted, an installation as hereinbefore disclosed with twin adjustable-cooling circuits will permit, by a method included in the scope ofthis invention (maintenance of the walls of the blenders annular chamber at an operating temperature below a given index temperature), of obtaining an improved, absolutely homogeneous foam.

It goes without saying that many changes may be made in the exemplary forms of embodiments hereinbefore described without departing from the scope of the invention.

What is claimed is:

I. An installation for making foams, notably plastics-based foams, comprising, for the purpose of incorporating air into the foam-producing mixture a motor-driven blender comprising an annular mixing chamber of revolution having an upstream end and a downstream end, means for introducing a foam producing mixture, and air, into the upstream end of the blender and circulating the mixture including air from the upstream end to the downstream end thereof, said annular mixing chamber being bounded by respectively outer and inner coaxial walls defined by surfaces of revolution, means for subjecting said outer wall and said inner wall to the actions of separate respective cooling circuits each equipped with individual adjustment means for adjusting its cooling effectiveness, one said adjustment means being operable to adjust the cooling power of its cooling circuit to a value different from the cooling power of the cooling circuit controlled by the other adjustments means, means for circulating a coolant fluid in each cooling circuit, said walls being impermeable so that the coolant fluid is kept separated from contact with said foam mixture, said two cooling circuits and the individual adjustment means thereof being arranged to maintain said outer and inner walls substantially at the same operating temperature within the full speed range of said blender, said operating temperature being lower than a critical index temperature above which the physical properties of the mixture are afiected, whereby notwithstanding the differing respective heat transmitting characteristics of the outer and inner walls, radial heat temperature gradients in the mixture are substantially avoided.

2. An installation according to claim 1, wherein each said cooling circuit individual adjustment means comprises temperature sensing means.

3. An installation according to claim 1, wherein the overall cooling system comprises a cold-generating unit supplying a coolant fluid to a pump the delivery conduit of which delivers said fluid in parallel to said two cooling circuits respectively associated with said outer wall and said inner wall and said individual adjustment means comprise flow regulating means.

4. An installation according to claim 3, wherein the blender has a hollow drive shaft, and the inner wall cooling circuit includes an inner feed conduit branching off the delivery conduit of said pump and connected to said shaft at the downstream end of the blender a first set of holes in said shaft providing access for the coolant to a cooling passage which is provided on the inside surface of said inner wall and a second set of holes, in said shaft providing access for the coolant to the shaft at the upstream end of said blender and thence, via a discharge conduit, with a recycling manifold, said inner wall cooling circuit adjustment means being a flow regulating valve connected into said inner feed conduit.

5. An installation according to claim 3, wherein the outer wall cooling circuit includes an outer feed conduit branching off the delivery conduit of said pump and connected at the downstream end of the blender to an outer wall cooling duct said duct being connected at the upstream end of said blender to a discharge conduit for returning the coolant fluid to a recycling manifold having a port at the inlet end of said coldgenerating unit, said outer wall cooling circuit adjustment means being a flow regulating valve connected into said outer feed conduit.

6. An installation according to claim 5, wherein said outer wall cooling duct is formed by a helicoid passage embodied in said outer wall.

7. A method for making improved plastics-based foam comprising the steps of introducing a foam-producing mixture, and air, into one end of an annular mixing chamber of revolution of a motor-driven blender, passing said mixture including air from the upstream end to the downstream end of said chamber, said annular mixing chamber being bounded by respectively outer and inner coaxial walls, subjecting the outer wall and the inner wall to the action of separate respective cooling circuits, each of which circuits is separately controlled such that the cooling power of one cooling circuit is different from the cooling power of the other cooling circuit, regulating the different cooling powers of the two cooling circuits to maintain the said inner wall and the said outer wall continuously at substantially the same operating temperature within the full speed range of the said blender, notwithstanding the differing heat transmitting characteristics of the outer and inner walls, said operating temperature being lower than a critical index temperature above which the physical properties of the mixture are affected, whereby radial temperature gradients in the mixture are substantially avoided, thereby providing a foam of improved homogeneity.

8. A method according to claim 7 for making plastics-based foams embodying at least one resin and containing a stabilizing agent, wherein the critical index temperature is the pregelling temperature of the resin and lies within the temperature limits between which said stabilizing agent is effective.

9. A method according to claim 8, wherein said resin is a polyvinyl plastisol and the critical index temperature is 27 C. and the operating temperature is above 22 C.

10. An installation for making foams, notably plastics-based foams, comprising, for the purpose of incorporating air into the foam-producing mixture, a motor driven blender comprising an annular mixing chamber of revolution through which said mixture circulates from the upstream end to the downstream end thereof, said annular mixing chamber being bounded by two respectively outer and inner coaxial walls preferably bearing protuberances thereon and possessed of relative rotating motion, said outer wall and said inner wall being respectively subjected to the actions of two separate cooling circuits each equipped with individual adjustment means for adjusting its effectiveness, said two cooling circuits where Tis the blending time, D the foam output rate, and S is the annular cross section of said mixing chamber, or the mean annular cross section in the case of chamber sections formed by said divergent and convergent sections. 

2. An installation according to claim 1, wherein each said cooling circuit individual adjustment means comprises temperature sensing means.
 3. An installation according to claim 1, wherein the overall cooling system comprises a cold-generating unit supplying a coolant fluid to a pump the delivery conduit of which delivers said fluid in parallel to said two cooling circuits respectively associated with said outer wall and said inner wall and said individual adjustment means comprise flow regulating means.
 4. An installation according to claim 3, wherein the blender has a hollow drive shaft, and the inner wall cooling circuit includes an inner feed conduit bRanching off the delivery conduit of said pump and connected to said shaft at the downstream end of the blender a first set of holes in said shaft providing access for the coolant to a cooling passage which is provided on the inside surface of said inner wall and a second set of holes, in said shaft providing access for the coolant to the shaft at the upstream end of said blender and thence, via a discharge conduit, with a recycling manifold, said inner wall cooling circuit adjustment means being a flow regulating valve connected into said inner feed conduit.
 5. An installation according to claim 3, wherein the outer wall cooling circuit includes an outer feed conduit branching off the delivery conduit of said pump and connected at the downstream end of the blender to an outer wall cooling duct, said duct being connected at the upstream end of said blender to a discharge conduit for returning the coolant fluid to a recycling manifold having a port at the inlet end of said cold-generating unit, said outer wall cooling circuit adjustment means being a flow regulating valve connected into said outer feed conduit.
 6. An installation according to claim 5, wherein said outer wall cooling duct is formed by a helicoid passage embodied in said outer wall.
 7. A method for making improved plastics-based foam comprising the steps of introducing a foam-producing mixture, and air, into one end of an annular mixing chamber of revolution of a motor-driven blender, passing said mixture including air from the upstream end to the downstream end of said chamber, said annular mixing chamber being bounded by respectively outer and inner coaxial walls, subjecting the outer wall and the inner wall to the action of separate respective cooling circuits, each of which circuits is separately controlled such that the cooling power of one cooling circuit is different from the cooling power of the other cooling circuit, regulating the different cooling powers of the two cooling circuits to maintain the said inner wall and the said outer wall continuously at substantially the same operating temperature within the full speed range of the said blender, notwithstanding the differing heat transmitting characteristics of the outer and inner walls, said operating temperature being lower than a critical index temperature above which the physical properties of the mixture are affected, whereby radial temperature gradients in the mixture are substantially avoided, thereby providing a foam of improved homogeneity.
 8. A method according to claim 7 for making plastics-based foams embodying at least one resin and containing a stabilizing agent, wherein the critical index temperature is the pregelling temperature of the resin and lies within the temperature limits between which said stabilizing agent is effective.
 9. A method according to claim 8, wherein said resin is a polyvinyl plastisol and the critical index temperature is 27* C. and the operating temperature is above 22* C.
 10. An installation for making foams, notably plastics-based foams, comprising, for the purpose of incorporating air into the foam-producing mixture, a motor driven blender comprising an annular mixing chamber of revolution through which said mixture circulates from the upstream end to the downstream end thereof, said annular mixing chamber being bounded by two respectively outer and inner coaxial walls preferably bearing protuberances thereon and possessed of relative rotating motion, said outer wall and said inner wall being respectively subjected to the actions of two separate cooling circuits each equipped with individual adjustment means for adjusting its effectiveness, said two cooling circuits and the individual adjustment means thereof being arranged so as to enable said outer and inner walls to be maintained substantially at the same operating temperature within the full speed range of said blender, said operating temperature being lower than a critical index temperature above with the pHysical properties of the mixture are affected, wherein said blender is formed by divergent sections and convergent sections and at least one interconnecting cylindrical section and the axial length H of each section is given by the formula: where T is the blending time, D the foam output rate, and S is the annular cross section of said mixing chamber, or the mean annular cross section in the case of chamber sections formed by said divergent and convergent sections. 